1. Field of the Invention
The present invention relates to a laser maintaining and repairing apparatus for processing a material, improving the residual stress of the surface of a workpiece, modifying the surface of the workpiece and welding and repairing by a laser beam, and more particularly to a laser maintaining and repairing apparatus capable of efficiently emitting a laser beam of high energy density, which generates the laser beam by a laser oscillator and guides and converges the beam to the portion to be processed at a predetermined distance.
2. Prior Art
A laser beam is a coherent monochromatic light having a very short wavelength, well aligned phases and every high convergence. Accordingly, since the laser beam can be concentrated at the light energy of high density to a narrow area, it is utilized in a wide field such as a communication, a measurement, a medical treatment and metallic machining.
In the field of the metallic machining, various machining such as welding, cutting, perforating and surface modifying are conducted by utilizing the laser beam of high energy density. Since the processing using the laser beam is non-contact processing, any place can be processed if the beam can arrive at the place. And, a laser emitting head for converging to emit the laser beam to a portion to be processed can be reduced in size. Since the heat input to a material is small, it has a merit of the possibility of precise processing. The processing using the laser beam is utilized for the maintenance and repair of a nuclear pressure vessel and a structure in a nuclear reactor for a nuclear power plant by applying these merits.
For example, the structure in a boiling water reactor is formed of a material having sufficient corrosion resistance and high temperature strength in a high temperature and high pressure environment such as, for example, austenite stainless steel or nickel-based alloy. These members are exposed with severe environment for a long time by the operation of a plant for a long period, and affected by neutron radiation, and hence the problem of the material deterioration must be considered. Particularly, the vicinity of the welded portion of the structure in the reactor has a potential danger of stress corrosion cracking due to the sensitization and tensile residual stress of the material due to the welding input heat.
Recently, various material surface improving technology has been developed as preventive maintenance remedy for the safety operation of a nuclear power plant. Of them, technique for emitting a laser beam to the surface of a material to modify the surface is disclosed, for example, in Japanese Patent Publication No. 246483/1995 and 206869/1996.
The former relates to a method for emitting a laser beam from a pulse laser to the surface (to be processed) of a workpiece via a reflecting mirror to retain a compression stress at the surface to be processed by emitting the beam to the surface to be processed while altering the emitting position.
The latter relates to a method for emitting a laser beam of high power and short pulse having a visible wavelength to the surface to be processed dipped in cooling water to improve the residual stress of the surface to be processed and to remove the crack or clad.
In these methods and apparatuses, an optical fiber is used as beam guiding means for guiding a laser beam from an oscillator to the position of the surface to be processed. The optical fiber has merits of flexibility and possibility of guiding the beam to a narrow portion.
In the laser beam emitting head, the beam guided via the optical fiber is converged to an arbitrary area via a condensing optical system such as a lens, further altered at its optical path by a mirror, and emitted to an arbitrary place to be processed
As described above, the conventional laser processing apparatus, in the case that the laser oscillator and the surface to be processed are located at a distance, transfers the laser beam generated from the laser oscillator via the beam guiding means such as the optical fiber, converges the laser beam by the lens and the reflecting mirror at the end of the beam guide passage (laser beam transfer route), and emits the laser beam to the portion to be processed.
FIG. 20 is a structural view of a conventional laser processing apparatus of the case that a laser oscillator and a surface to be processed are separate.
As shown in FIG. 20, an emitting head 51 of the laser processing apparatus installed in liquid has a condensing lens 53 for converging a laser beam 52 transmitted from a laser light source (laser oscillator), not shown in FIG. 20, via beam guiding means (optical fiber), a reflecting mirror 55 for reflecting the condensed beam 52 to emit the beam 52 to a portion 54 to be processed, and an optical window 56 for sealing liquid to hold the condensing lens 53 and the reflecting mirror 55 in the air (in the beam guiding means of a liquid-tight structure) to enable the work in the liquid.
The laser beam used for the processing has a high energy density. When the portion 54 to be processed and the emitting head 51 are near at hand, the laser beam become the high energy density even on the surfaces of the optical window 56 and the reflecting mirror 55. When the pulse energy of the laser beam transmitted is 200 mJ and the emitting area on the surface of the material is 1 mm.sup.2, the energy density arrives at 20J/cm.sup.2. In case that the portion 54 to be processed and the optical window 56 are close to one another, when the energy density is 20J/cm.sup.2 on the surface of the optical window 56 and the beam diameter is 5 mm.sup.2, the optical strength of 4J/cm.sup.2 or more is required on the surface of the reflecting mirror 55.
However, in the conventional laser processing apparatus of the constitution described above, there is a limit to emit the laser beam of the high energy density due to the restrictions in the optical fiber for guiding the beam, and the lens and the reflecting mirror for concentrating or reflecting the laser beam. Further, there is a difficulty of processing due to the use of the optical fiber and the lens and the reflecting mirror of the above-described constitution. This will be described below.
The conventional laser processing method and apparatus as described above use the optical fiber to guide the laser beam from the oscillator to the position of the surface to be processed, but there is a limit in the power or energy capable of guiding the laser beam via the optical fiber. The material limit of the optical fiber limits the energy density of the laser beam of the conventional laser processing apparatus, and therefore limits the processing performance.
In the case for the purpose of modifying the surface of a material as described above, to obtain the optical power and energy density at the necessary surface to be processed, it is necessary to concentrate the beam emitted from the optical fiber as small as possible. However, the beam emitted from the optical fiber is substantially equivalent to the diffused beam with the core of the optical fiber as a light source, and has properties that the beam is expanded and emitted according to the NA (Numerical Aperture) number determined in accordance with the refractive index of the optical fiber constituting material. In general, the optical fiber for power laser has NA number of about 0.2. The beam emitted from the optical fiber is diffused at an angle of approximately 46 degrees (=2 sin.sup.-1 (0.2)).
To emit the beam from such optical fiber to the surface to be processed, it is necessary to focus the image of the core of the optical fiber by using a lens. In this case, the size of the image on the surface to be processed becomes the product of multiplying the magnifying ratio by the size of the optical fiber core. From such properties, it is necessary to reduce the magnifying ratio to converge the beam emitted from the optical fiber as small as possible.
However, if the magnifying ratio is reduced, the NA number of the converged beam emitted from the lens is increased. That is, when the magnifying ratio is M, the NA number of the optical fiber is A and the NA number of the focusing side is B, there is the relationship of M=A/B. For example, when the core of the optical fiber is the same by using the optical fiber (A=0.2), B=0.2 is obtained, and the beam is converged at an angle of 46 degrees.
In general, when the laser beam is converted such a deep angle, the focal depth becomes very shallow. Accordingly, in the case that the surface to be processed with a complicated shape is an object, an emitting optical system having an optical fiber and a lens must be precisely positioned.
The above-mentioned subject is always accompanied with the case that the beam is transmitted via the optical fiber, and it is not corresponded when the beam is transmitted via a space and the beam is converged with the lens having low NA number.
On the other hand, the emitting head for converging and reflecting the laser beam to emit the beam to the surface to be processed also has a restriction for emitting the laser beam of high energy density.
As described above, the laser beam transmitted from the laser beam source to the emitting head near the portion to be processed is converged to an arbitrary area by a converging optical system such as a lens, further altered at the optical path by a reflecting mirror, emitted to an arbitrary place to be processed, and processed, but when the laser beam of high energy density is emitted, to prevent the damage of the optical system such as a condensing lens and a reflecting mirror, it is necessary to incorporate the optical strength capable of sufficiently enduring against the power of the laser beam.
Particularly, in the case that the reflecting mirror for reflecting the laser beam after the convergence and the emitting head containing the optical system is required to be liquid-tight due to the reason of underwater processing, the optical window for externally emitting the laser beam from the interior of the emitting head must endure against the laser beam of very high energy density.
However, the reflecting film of dielectric material normally formed on the surface of a reflecting mirror has high reflectivity but expensive cost, and the optical strength is more or less deteriorated. In the case of the laser of continuous beam, the energy density of the laser beam normally does not arrive at the level of the degree that these optical components are damaged, but in the case of the laser beam of very short pulse, the peak energy might arrive at the level or higher, and it becomes the cause for limiting the energy of the laser beam capable of being used for processing.
Since the reflecting mirror is easily damaged, in the laser processing apparatus of FIG. 20, for example, the incident angle to the reflecting mirror 55 is limited to the determined angle such as 0.degree. or 45.degree.. Therefore, it cannot to emit the laser beam by driving the mirror to an arbitrary angle. Thus, the fine adjustment at the time of processing is limited.
Further, when the converging lens 53 and the reflecting mirror 55 are located near to each other to increase the beam diameter of the laser beam on the surface of the reflecting mirror 55, the converging lens 53 having a short focal length must be used. The problem that the beam diameter on the reflecting mirror must be increased causes that the focal length of the lens must be further shortened together with the problem that the magnifying ratio due to the above-descried optical fiber must be reduced, thereby increasingly causing the error in the emitting area on the surface of the emitting portion.